Rodents and primates have been used as models for human behavior. Studies in these species are limited in their ability to determine why sexual and aggressive behaviors are organized to a greater degree in male mammals. Two alternate explanations of this pattern are that 1) the sex determining mechanism (XY males) causes organization or 2) that organization is evolutionarily adaptive. These hypotheses make different predictions for organization in different vertebrates. These alternate predictions will be tested in gonadectomized male and female lizards from two egg incubation temperatures. Geckos will be implanted with Silastic tubing containing cholesterol, dihydrotestosterone, testosterone, or estradiol. Patterns of male- and female-typical behavior exhibited by these groups when treated with sex steroids will distinguish among the following hypotheses: 1) there is no organization, 2) organization is caused by incubation temperature, 3) one sex is organized by incubation temperature and the other sex is unorganized, 4) one sex is organized as in mammals (or birds), or 5) both sexes are organized. Also hypothesized, 1) there are steroid-induced changes in molecular neuroendocrinology as measured by in situ hybridization of mRNA and 2) changes are correlated with behavioral organization. Hormone treatments that activate male-typical behavior may affect androgen receptor (AR) mRNA, estrogen receptor (ER) mRNA, progesterone (PR) and/or aromatase mRNA levels in the preoptic area of the brain. Hormone treatments that activate female-typical behavior may affect AR, ER, and PR mRNA levels and possibly aromatase mRNA in the ventromedial hypothalamus. Hormone treatments that activate aggression may affect AR, ER, PR, and/or aromatase mRNA levels in the amygdala. The bases of sexual and aggressive behavior in this species may provide insight to mechanisms of "abnormal" behavior in humans. For example, aberrant organization (via genetic and/or environmental causes) in humans could result in "abnormal" behavior.